In one type of thermal printer which prints colored images, a carrier contains a repeating series of spaced frames of different colored heat transferable dyes. In such apparatus, the carrier is disposed between a receiver, such as coated paper, and a print head formed of, for example, a plurality of individual heating elements. When a particular heating element is energized, it is heated and causes dye from the carrier to transfer to the receiver. The density or darkness of the printed color dye is a function of the energy delivered from the heating element to the carrier.
Thermal dye transfer printers offer the advantage of true "continuous tone" dye density transfer. This result is obtained by varying the energy applied to each heating element, yielding a variable dye density image pixel on the receiver.
One way to provide energy to the heating elements is to provide a constant current source for each heating element. The amount of energy delivered is a function of the time the current is applied to a heating element. Such an arrangement is called pulse width modulation.
Another type of thermal printer uses resistive ribbons. In U.S. Pat. No. 4,434,356 assigned to the IBM Corporation, a circuit is described which provides constant current to a resistive ribbon printer. Resistive ribbon printing technology uses a ribbon including a metal conducting layer and a dye layer containing dye to be transferred to a receiver. Current is supplied to the resistive ribbon by an electrode or array of electrodes and returns to ground via the conductive layer. The electric current is converted to heat through the resistive heating of the ribbon. The heat causes dye to transfer to a receiver.
This circuit is used in an application which is essentially a binary process. That is, the desired effect is to either transfer dye or ink or not to transfer dye. Up to the present invention, resistive ribbon technology has not been used to provide continuous tone prints because of the requirement that variable, precisely controlled, heat levels be applied to a transferable dye.